fat: don't use obsolete random32 call in namei_vfat

Introduced in commit 16b6ba6d7f6b0e8a2baeb14b128df0bb0a0a5680
("FAT: Add CONFIG_VFAT_FS_NO_DUALNAMES option"), the use of
random32 is no longer allowed.

The compatibility wrappers have been removed, so we have to
align the additions from this older patch to match upstream
commit 496f2f93 ("random32: rename
random32 to prandom")  which does:

    This renames all random32 functions to have 'prandom_' prefix as follows:

      void prandom_seed(u32 seed);      /* rename from srandom32() */
      u32 prandom_u32(void);            /* rename from random32() */

    [...]

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>